what is the cause for sharp shadows when using strong bump?
It really depends on how you're applying your bump. 

If you're doing it via the new "True" displacement (or "Both") settings in Cycles materials, your mesh should be adequately subdivided in order for it to work properly. The displacement is indeed applied on the actual geometry (vertices) of the mesh, not on its faces. 

To do so, try using the Subdivision Surface modifier with an adequate level of subdivision or, if you're using the experimental features on a recent version of Blender, you can make it adaptive: reference.

Also, increasing the lamp size parameter could help.





(the black spots in the last one are because I didn't put too much care in creating a seamless UV map, and a fine subdivision, together with a strong bump value, makes it more evident)



Related: Blender Cycles True Displacement